In the late afternoon of November 1, 1950 thousands of Chinese soldiers, accompanied by eerie bugle calls, descended from the hills near Unsan, North Korea throwing hand grenades and firing burp guns at scattered units of the U.S. 8th Cavalry Regiment, 1st Cavalry Division and South Korean ROK forces moving deep into North Korea.
Within hours the ROK 15th Regiment on the 8th Cavalry’s right flank collapsed, while the 1st and 2d Battalions of the 8th Cavalry fell back in disarray into the city of Unsan.
Wikipedia
On the morning of November 2 men of the US 8th Cavalry attempting to withdraw met a Chinese roadblock that forced them to abandon their artillery and take to the hills in small groups. Only a few scattered survivors made it back to UN lines.
Other elements of the 1st Cavalry Division tried unsuccessfully to reach the isolated battalion with a two-battalion counterattack on the dug-in Chinese positions encircling the 8th Cavalry, but were unable to break through the Chinese line.
At twilight with the relief effort unsuccessful, survivors broke into small groups to retreat under cover of darkness—most did not make it and >800 hundred men of the 8th Cavalry were lost.
The massive Chinese attacks shocked the UN forces. After the Incheon landing and breakout from the Pusan Perimeter, the war seemed to have been won. But now the Chinese had entered the war in force.